U.S. patent application number 13/852447 was filed with the patent office on 2013-11-07 for carrier detection and parallel gsm cell search in multimode terminals.
This patent application is currently assigned to ST-Ericsson SA. The applicant listed for this patent is ST-ERICSSON SA. Invention is credited to Sajal Kumar Das, Naveen Jacob, Ravi Jandial, Ivine Kuruvila.
Application Number | 20130295925 13/852447 |
Document ID | / |
Family ID | 46196985 |
Filed Date | 2013-11-07 |
United States Patent
Application |
20130295925 |
Kind Code |
A1 |
Jacob; Naveen ; et
al. |
November 7, 2013 |
Carrier Detection and Parallel GSM Cell Search in Multimode
Terminals
Abstract
A wireless communication apparatus is arranged to detect, among
a plurality of modulated carrier signals of different frequencies,
at least one of the modulated carrier signals modulated with a tone
burst. A receiver provides a composite signal comprising the
plurality of modulated carrier signals received simultaneously. An
ADC generates samples of the composite signal, and the samples of
the composite signal are divided into a plurality of blocks. The
samples of each block are transformed into frequency domain
components, and the frequency domain components of each block are
divided into a plurality of groups, each group corresponding to a
range of frequencies occupied by a different one of the modulated
carrier signals. Tone burst detection is performed on each group,
and it is determined which of the modulated carrier signals is
modulated with the tone burst, according to which of the groups the
tone burst is detected in.
Inventors: |
Jacob; Naveen; (Kottayam,
IN) ; Das; Sajal Kumar; (Bangalore, IN) ;
Jandial; Ravi; (Bangalore, IN) ; Kuruvila; Ivine;
(Kottayam, IN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ST-ERICSSON SA |
Plan-les-Ouates |
|
CH |
|
|
Assignee: |
ST-Ericsson SA
Plan-les-Ouates
CH
|
Family ID: |
46196985 |
Appl. No.: |
13/852447 |
Filed: |
March 28, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61620196 |
Apr 4, 2012 |
|
|
|
Current U.S.
Class: |
455/434 |
Current CPC
Class: |
H04J 11/0069 20130101;
H04J 11/0089 20130101; H04L 27/265 20130101; H04W 48/16
20130101 |
Class at
Publication: |
455/434 |
International
Class: |
H04W 48/16 20060101
H04W048/16 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 30, 2012 |
EP |
12162785.5 |
Claims
1. A method of detecting, among a plurality of modulated carrier
signals of different frequencies, at least one of the carrier
signals modulated with a tone burst having a tone burst duration,
wherein the modulated carrier signals are Global System for Mobile
Communications (GSM) carrier signals and the tone burst is
contained within a GSM Frequency Correction Channel (FCCH), the
method comprising: generating a composite signal comprising the
plurality of modulated carrier signals received simultaneously,
using a receiver adapted to receive signals in accordance with the
Third Generation Partnership Project Long Term Evolution
specifications; generating samples by sampling the composite
signal; dividing the samples of the composite signal into a
plurality of blocks, each of the blocks comprising a number of
samples corresponding to not more than half of the tone burst
duration; transforming the samples of each block into frequency
domain components; dividing the frequency domain components of each
block into a plurality of groups, each group corresponding to a
range of frequencies occupied by a different one of the modulated
carrier signals; performing tone burst detection on each group; and
determining which of the carrier signals is modulated with the tone
burst, according to which of the groups the tone burst is detected
in.
2. The method of claim 1 wherein the tone burst occurs at time
intervals of not less than a tone burst interval, and the method
comprises receiving the plurality of modulated carrier signals
simultaneously for a time period not less than the tone burst
interval plus the tone burst duration.
3. The method of claim 1 wherein transforming the samples of each
block into frequency domain components comprises performing a Fast
Fourier Transform on the samples.
4. The method of claim 1 wherein performing tone burst detection
comprises: convolving the frequency domain components with their
mirror-imaged conjugate; and determining that a carrier signal is
modulated by the tone burst if the convolved frequency domain
components comprise a direct current (DC) component exceeding a
threshold.
5. The method of claim 4 wherein the threshold is dependent on a
mean of the convolved frequency domain components excluding the DC
component.
6. The method of claim 4, further comprising normalizing the
frequency domain components within each group prior to the
convolving.
7. The method of claim 6 wherein normalizing the frequency domain
components comprises dividing each frequency domain component
within each group by the root mean square of the frequency domain
components within each respective group.
8. The method of claim 1, further comprising determining a received
signal strength indication from the frequency domain components of
the group corresponding in frequency to each of the carrier signals
determined to be modulated with the tone burst.
9. The method of claim 1, further comprising: determining, for at
least one of the detected tone bursts, a tone frequency, the tone
frequency being a frequency of the frequency domain component
having the largest magnitude within the respective group in which
the tone burst is detected; and estimating a frequency error
dependent on a difference between the determined tone frequency and
a frequency of the carrier signal modulated with the tone
burst.
10. The method of claim 1, further comprising determining a start
position of a GSM frame from a position, within the plurality of
blocks, of a block comprising the group in which the tone burst is
detected.
11. A wireless communication apparatus arranged to detect, among a
plurality of modulated carrier signals of different frequencies, at
least one of the modulated carrier signals modulated with a tone
burst having a tone burst duration, wherein the modulated carrier
signals are Global System for Mobile Communications (GSM) carrier
signals and the tone burst is contained within a GSM Frequency
Correction Channel (FCCH), the wireless communication apparatus
comprising: a receiver adapted to receive signals in accordance
with the Third Generation Partnership Project Long Term Evolution
specifications and arranged to provide a composite signal
comprising the plurality of modulated carrier signals received
simultaneously; an analog-to-digital converter arranged to generate
samples of the composite signal; a block formation stage arranged
to divide the samples of the composite signal into a plurality of
blocks, each of the blocks comprising a number of samples
corresponding to not more than half of the tone burst duration; a
transform stage arranged to transform the samples of each block
into frequency domain components; a group formation stage arranged
to divide the frequency domain components of each block into a
plurality of groups, each group corresponding to a range of
frequencies occupied by a different one of the modulated carrier
signals; a tone burst detection stage arranged to perform tone
burst detection on each group; and a recording stage arranged to
record which of the modulated carrier signals is modulated with the
tone burst, according to which of the groups the tone burst is
detected in.
12. The apparatus of claim 11 wherein the tone burst occurs at time
intervals of not less than a tone burst interval, and the tone
burst detection stage is arranged to receive the plurality of
modulated carrier signals simultaneously for a time period not less
than the tone burst interval plus the tone burst duration.
13. The apparatus of claim 11 wherein the transform stage is
arranged to transform the samples of each block into frequency
domain components by performing a Fast Fourier Transform on the
samples.
14. The apparatus of claim 11 wherein the tone burst detection
stage is arranged to perform tone burst detection on each group by:
convolving the frequency domain components with their mirror-imaged
conjugate; and determining that a carrier signal is modulated by
the tone burst if the convolved frequency domain components
comprise a direct current (DC) component exceeding a threshold.
15. The apparatus of claim 14 wherein the threshold is dependent on
a mean of the convolved frequency domain components excluding the
DC component.
16. The apparatus of claim 14 wherein the tone burst detection
stage is further arranged to normalize the frequency domain
components within each group prior to the convolving.
17. The apparatus of claim 16 wherein normalizing the frequency
domain components comprises dividing each frequency domain
component within each group by the root mean square of the
frequency domain components within each respective group.
18. The apparatus of claim 11, further comprising a signal strength
stage arranged to determine a received signal strength indication
from the frequency domain components of the group corresponding in
frequency to each of the carrier signals determined to be modulated
with the tone burst.
19. The apparatus of claim 11, further comprising a frequency error
stage arrange to determine, for at least one of the detected tone
bursts, a tone frequency, the tone frequency being a frequency of
the frequency domain component having the largest magnitude within
the respective group in which the tone burst is detected; and
estimate a frequency error dependent on a difference between the
determined tone frequency and a frequency of the carrier signal
modulated with the tone burst.
20. The apparatus of claim 11, further comprising a framing stage
arranged to determine a start position of a GSM frame from a
position, within the plurality of blocks, of a block comprising the
group in which the tone burst is detected.
21. A nontransient machine-readable medium storing computer program
code arranged for, when executed on a processor, processing samples
of a composite signal comprising a plurality of modulated carrier
signals of different frequencies according to the following steps,
wherein the modulated carrier signals are Global System for Mobile
Communications (GSM) carrier signals and at least one of the
carrier signals is modulated with a tone burst having a tone burst
duration and contained within a GSM Frequency Correction Channel
(FCCH): dividing the samples of the composite signal into a
plurality of blocks, each of the blocks comprising a number of
samples corresponding to not more than half of the tone burst
duration; transforming the samples of each block into frequency
domain components; dividing the frequency domain components of each
block into a plurality of groups, each group corresponding to a
range of frequencies occupied by a different one of the modulated
carrier signals; performing tone burst detection on each group; and
determining which of the carrier signals is modulated with the tone
burst, according to which of the groups the tone burst is detected
in.
Description
[0001] This application claims priority to U.S. Provisional Patent
Application Ser. No. 618/620,196, titled "Carrier Detection and
Parallel GSM Cell Search in Multimode Terminals," filed Apr. 4,
2012, the disclosure of which is incorporated herein by reference
in its entirety.
[0002] This application claims priority to European Patent
Application No. EP 12162785.5, filed Mar. 30, 2012, the disclosure
of which is incorporated herein by reference in its entirety.
FIELD OF INVENTION
[0003] The present invention relates generally to wireless
telecommunications, and in particular to an apparatus, method, and
computer program code implementing a method of detecting, among a
plurality of modulated radio frequency carrier signals of different
frequencies, at least one of the carrier signals modulated with a
tone burst.
BACKGROUND
[0004] A typical wireless communication apparatus, such as a mobile
terminal, is required to scan a range of radio frequencies in order
to identify available communication systems and radio frequency
(RF) carrier signals. In order to decode information transmitted by
a communication system, the mobile terminal is required to
synchronize to the system, for which purpose synchronization
information is transmitted by base stations of the communication
system.
[0005] For example, in a communications system operating in
accordance with the GSM standard, a base station in each cell
transmits, on a broadcast control channel (BCCH), a frequency
correction channel (FCCH) and a synchronization channel (SCH). The
FCCH enables a mobile terminal to synchronize its local oscillator
to the base station and then to receive the synchronization channel
(SCH), which provides further synchronization information. The FCCH
consists solely of a tone burst at 67.7 kHz relative to a carrier
center frequency. On those carrier signals conveying the FCCH, the
FCCH is transmitted once every 10 frames, corresponding to a rate
of once every 46.15 ms, and the tone burst has a duration of 576.92
.mu.s.
[0006] After initial power-on, a GSM mobile terminal typically
tunes sequentially to available RF carrier signals that could
potentially convey the broadcast channel of suitable cells, and
measures the received signal strength of each of these channels.
Each time that a carrier signal having a sufficient received signal
strength is detected, the mobile terminal attempts to detect the
FCCH and synchronize the frequency of its local oscillator to the
carrier signal, and then attempts to detect the SCH and synchronize
to the GSM fames and time slots, followed by decoding broadcast
system information to check the suitability of the cell.
[0007] A multi-mode mobile terminal, capable of operating in
accordance with more than one radio access technology (RAT), is
potentially required to measure the received signal strength and
perform cell selection for each carrier signal of each RAT. For
example, the mobile terminal may evaluate carriers for each RAT in
an order of preference until a suitable cell is identified, such as
a fourth generation (4G) technology first, followed by a third
generation (3G) technology, and finally a second generation (2G)
technology. Such a process can result in an undesirably long delay
before a cell is selected.
[0008] The Background section of this document is provided to place
embodiments of the present invention in technological and
operational context, to assist those of skill in the art in
understanding their scope and utility. Unless explicitly identified
as such, no statement herein is admitted to be prior art merely by
its inclusion in the Background section.
SUMMARY
[0009] The following presents a simplified summary of the
disclosure in order to provide a basic understanding to those of
skill in the art. This summary is not an extensive overview of the
disclosure and is not intended to identify key/critical elements of
embodiments of the invention or to delineate the scope of the
invention. The sole purpose of this summary is to present some
concepts disclosed herein in a simplified form as a prelude to the
more detailed description that is presented later.
[0010] According to one or more embodiments described and claimed
herein, a plurality of modulated carrier signals of different
frequencies is simultaneously received. The signals are digitized
and divided into blocks. Signal samples from each block are
transformed into the frequency domain, and the frequency domain
components are formed into a plurality of groups, each
corresponding to a range of frequencies occupied by a different one
of the modulated carrier signals. Tone burst detection is performed
on each group, and the results recorded. The detection of a tone
burst indicates, e.g., a GSM FCCH signal. The received signal
strength of the detected tone signals may be compared, to select
candidate carrier frequencies on which to synchronize and decode
broadcast system information.
[0011] One embodiment relates to a method of detecting, among a
plurality of modulated carrier signals of different frequencies, at
least one of the carrier signals modulated with a tone burst having
a tone burst duration, wherein the modulated carrier signals GSM
carrier signals and the tone burst is contained within a GSM FCCH.
A composite signal comprising the plurality of modulated carrier
signals received simultaneously is generated, using a receiver
adapted to receive signals in accordance with the 3GPP LTE
specifications. Samples are generated by sampling the composite
signal. The samples of the composite signal are divided into a
plurality of blocks, each of the blocks comprising a number of
samples corresponding to not more than half of the tone burst
duration. The samples of each block are transformed into frequency
domain components. The frequency domain components of each block
are divided into a plurality of groups, each group corresponding to
a range of frequencies occupied by a different one of the modulated
carrier signals. Tone burst detection is performed on each group. A
determination of which of the carrier signals is modulated with the
tone burst is made, according to which of the groups the tone burst
is detected in.
[0012] Another embodiment relates to a wireless communication
apparatus arranged to detect, among a plurality of modulated
carrier signals of different frequencies, at least one of the
modulated carrier signals modulated with a tone burst having a tone
burst duration, wherein the modulated carrier signals are GSM
carrier signals and the tone burst is contained within a GSM FCCH.
The wireless communication apparatus includes a receiver adapted to
receive signals in accordance with the 3GPP LTE specifications and
arranged to provide a composite signal comprising the plurality of
modulated carrier signals received simultaneously. The apparatus
also includes an analog-to-digital converter arranged to generate
samples of the composite signal, and a block formation stage
arranged to divide the samples of the composite signal into a
plurality of blocks, each of the blocks comprising a number of
samples corresponding to not more than half of the tone burst
duration. The apparatus further includes a transform stage arranged
to transform the samples of each block into frequency domain
components, and a group formation stage arranged to divide the
frequency domain components of each block into a plurality of
groups, each group corresponding to a range of frequencies occupied
by a different one of the modulated carrier signals. The apparatus
still further includes a tone burst detection stage arranged to
perform tone burst detection on each group, and a recording stage
arranged to record which of the modulated carrier signals is
modulated with the tone burst, according to which of the groups the
tone burst is detected in.
[0013] Yet another embodiment relates to a nontransient
machine-readable medium storing computer program code arranged for,
when executed on a processor, processing samples of a composite
signal comprising a plurality of modulated carrier signals of
different frequencies according to the following steps, wherein the
modulated carrier signals are GSM carrier signals and at least one
of the carrier signals is modulated with a tone burst having a tone
burst duration and contained within a GSM FCCH. The steps performed
by the computer program code comprise dividing the samples of the
composite signal into a plurality of blocks, each of the blocks
comprising a number of samples corresponding to not more than half
of the tone burst duration; transforming the samples of each block
into frequency domain components; dividing the frequency domain
components of each block into a plurality of groups, each group
corresponding to a range of frequencies occupied by a different one
of the modulated carrier signals; performing tone burst detection
on each group; and determining which of the carrier signals is
modulated with the tone burst, according to which of the groups the
tone burst is detected in.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The present invention will now be described more fully
hereinafter with reference to the accompanying drawings, in which
embodiments of the invention are shown. However, this invention
should not be construed as limited to the embodiments set forth
herein. Rather, these embodiments are provided so that this
disclosure will be thorough and complete, and will fully convey the
scope of the invention to those skilled in the art. Like numbers
refer to like elements throughout.
[0015] FIG. 1 is a flow chart depicting a method of detecting a
carrier signal modulated with a tone burst.
[0016] FIG. 2 is a functional block schematic diagram of a wireless
communication apparatus.
[0017] FIG. 3 is a graph depicting the probability of correct
detection of a tone burst for a range of signal to noise
ratios.
[0018] FIG. 4 is a graph illustrating magnitudes of the frequency
domain components for a signal to noise ratio of -20 dB.
[0019] FIG. 5 is graph illustrating autocorrelation of the
frequency domain components for a signal to noise ratio of -20
dB.
[0020] FIG. 6 is a graph illustrating magnitudes of the frequency
domain components where the carrier signals are not modulated with
a tone burst, for a signal to noise ratio of 0 dB.
[0021] FIG. 7 is graph illustrating autocorrelation of the
frequency domain components where the carrier signals are not
modulated with a tone burst, for a signal to noise ratio of 0
dB.
DETAILED DESCRIPTION
[0022] It should be understood at the outset that although
illustrative implementations of one or more embodiments of the
present disclosure are provided below, the disclosed systems and/or
methods may be implemented using any number of techniques, whether
currently known or in existence. The disclosure should in no way be
limited to the illustrative implementations, drawings, and
techniques illustrated below, including the exemplary designs and
implementations illustrated and described herein, but may be
modified within the scope of the appended claims along with their
full scope of equivalents.
[0023] A first embodiment of the method is described with reference
to GSM and 3GPP LTE, the latter being referred to simply as LTE, as
is customary. Referring to FIG. 1, a method comprises, at step 100,
receiving simultaneously a plurality of modulated carrier signals
of different frequencies. Each of the modulated carrier signals may
be a GSM carrier signal, the modulated carrier signals being spaced
apart by 200 kHz occupying a frequency range, that is, a bandwidth,
of 270.833 kHz. The plurality of GSM carrier signals may be
received simultaneously by an LTE receiver, an LTE receiver being a
receiver configured for receiving an LTE signal, which can be a
multi-carrier signal occupying 5 or 20 MHz. Therefore, up to 25 or
100 GSM carrier signals, consecutive in the frequency domain, may
be received simultaneously by an LTE receiver, although it is not
essential for the GSM carrier signals to be consecutive in the
frequency domain. The modulated carrier signals may be received for
a period of time that will ensure that a tone burst will have been
transmitted on those modulated carrier signals that convey the tone
burst periodically. Therefore, the receive period typically has a
minimum duration corresponding to the maximum time between the
start of one tone burst and the end of the next tone burst
transmitted by the same modulated carrier signal. In other words,
the receive period is not less than the interval at which the tone
burst is transmitted plus the tone burst duration. In the case of
GSM, therefore, the minimum receive period is 12 frames,
corresponding to a duration of 55.38 ms.
[0024] At step 110 the composite received signal, corresponding to
the plurality of modulated carrier signal received simultaneously,
is sampled to convert the composite signal to the digital domain. A
suitable sample rate for the LTE receiver is 30.72 M
samples/sec.
[0025] At step 120, the samples of the composite signal are divided
into blocks of consecutive samples. In order to ensure that, in at
least one of the blocks, the tone burst is present throughout the
block, each block may correspond to no more than half the tone
burst duration. In the case of GSM, where the FCCH has a duration
of 0.577 ms, and of reception by an LTE receiver employing a sample
rate of 30.72 M samples/sec, each block has no more than 17725
samples in this example. The block length may be, for example,
2.sup.13=8192 samples. This number, being power of two, enables
transformation to the frequency domain to be performed efficiently,
as described below.
[0026] At step 130, each of the blocks of samples is transformed to
the frequency domain. For this transformation, a Fast Fourier
Transform (FFT) may be used. The result of the transform is, for
each block, a plurality of frequency domain components of defined
magnitudes, each corresponding to a spectral bin of the FFT. With a
sample rate of 30.72 M samples/sec and an 8192-point FFT, the
frequency domain components are spaced by 30.72/8192 MHz, that is,
3.75 kHz.
[0027] At step 140, the frequency domain components for each block
are divided into groups. Each group is selected to correspond to a
range of frequencies occupied by a different one of the modulated
carrier signals. In the case of GSM, the center frequencies of the
modulated carrier signals are known, and may be stored, for example
in a look-up table, and the division of the frequency domain
components into blocks may align the center of each block with a
center frequency of a modulated carrier, except for an error of
+/-15 kHz attributed to a frequency error of a crystal or local
oscillator. For GSM, the center frequencies of the modulated
carrier signals are spaced by 200 kHz, and therefore, each group
may contain 53 of the frequency domain components spaced by 3.75
kHz.
[0028] Steps 150, 160 and 170 together constitute performing a tone
burst detection algorithm, in the frequency domain, on each group
of frequency domain components. At step 150, the frequency domain
components of each group are normalized. This may be performed by
dividing each frequency domain component within the group by the
root mean square (RMS) value of all of the frequency domain
components in the group. At step 160, for each group, a vector X(f)
formed by the normalized frequency domain components in the group
is convolved with its mirror-image conjugate X*(-f) multiplied by
e.sup.jWT, where w is frequency and T is a delay, to provide a
frequency domain indication of signal autocorrelation. The
frequency domain indication of signal autocorrelation may be
expressed as
R(f)=convolution(X(f), X*(-f)e.sup.jWT) (1)
In the context of GSM, T may be selected to be a delay
corresponding to 5 GSM symbols, this value being chosen to exceed
an RF front end filter delay in the LTE receiver, to avoid
introducing correlation in the RF front end filter.
[0029] When a group corresponds to a carrier signal modulated by a
tone burst, the result of the convolution, R(f), will have a strong
DC component, that is, the frequency domain component at DC, R(0),
will have a relatively high value. Therefore, at step 170, those
groups having a strong DC component are determined by comparing the
frequency domain component at DC with the average value of all
other frequency domain components in the group. In particular, the
ratio of the frequency domain component at DC to the average value
of all other frequency domain components in the group, which may be
expressed as R(0)/average(R(f.noteq.0)), may be calculated and
compared with a threshold. At step 180, the modulated carrier
signals that correspond to those groups that are determined, at
step 170, as having a strong DC component, and in particular for
which the above ratio exceeds the threshold, are recorded, for
example in a cell search list, these being considered as being
modulated with the tone burst. The value of the threshold may be
selected to suit operational requirements, a suitable value being
determined by means of computer simulation. In particular, the
threshold may be selected as a trade-off between the probability of
correctly detecting the tone burst and the probability of falsely
detecting a tone burst.
[0030] Having identified those modulated carrier signals that are
modulated by the tone burst, step 190 is an optional step which can
contribute further to synchronization. At step 190, an RSSI may be
determined for one or more of the modulated carrier signals that
are modulated with the tone burst. The RSSI may be determined as
the mean square value of the frequency domain components within a
group, without normalization. The RSSI may be used subsequently for
selecting a strong modulated carrier signal for further cell
selection steps. Also at step 190, a frequency error of the local
oscillator of the receiver, relative to one or more of the
modulated carrier signals, may be evaluated with a precision equal
to the frequency spacing between the frequency domain components in
each group, that is, with a resolution equal to the spacing of the
spectral bins of the FFT. In the case of a GSM modulated carrier
signal, this frequency error may be evaluated as
F.sub.ARFCN+67.71 kHz-F.sub.MAX (2)
where F.sub.ARFCN is the center frequency of the GSM modulated
carrier signal, such GSM carrier signals generally being identified
by an Absolute Radio Frequency Channel Number (ARFCN), and
F.sub.MAX is the frequency of the frequency domain component having
the largest absolute value in the group corresponding to the GSM
modulated carrier signal. The 67.71 kHz in equation (2) is the
baseband frequency of the tone burst in the FCCH. Also at step 190,
in the case of a GSM modulated carrier signal, the GSM frame and
slot boundaries may be determined, with a precision equal to one
block duration, that is, half the duration of the FCCH, according
to which of the blocks the tone burst is detected in, because the
FCCH containing the tone burst is transmitted in only the first
time slot of a GSM frame. In this way, frame and slot
synchronization may be provided.
[0031] Referring to FIG. 2, a wireless communication apparatus 200,
such as a mobile terminal or mobile phone, comprises an antenna 210
coupled to a duplex filter 220. An output of the duplex filter 220
is coupled to an input of a receiver 230 which is arranged to
receive a plurality of modulated carrier signals at different
frequencies simultaneously to provide a composite signal. The
receiver 230 may be, for example, an LTE receiver arranged to
receive a plurality of GSM modulated carrier signals
simultaneously. The composite signal, which comprises the plurality
of modulated carrier signals, is delivered at an output of the
receiver 230 that is coupled to an input of an analogue-to-digital
converter (ADC) 240 that samples the composite signal, delivering
samples of the composite signal at an output of the ADC 240. In the
case of an LTE receiver, the sample rate may be, for example, 30.72
M samples/sec. In the case of GSM modulated carrier signals, the
composite signal may be sampled for a period of time corresponding
to a minimum of 12 GSM frames, that is 55.38 ms, to ensure that an
FCCH will have been received if any of the GSM modulated carrier
signals are conveying a broadcast channel, BCCH. The output of the
ADC 240 is coupled to an input of a processing stage 250 for
processing the samples of the composite signal.
[0032] The processing stage 250 comprises a block formation stage
251, a transform stage 252, a group formation stage 253, a tone
burst detection stage 254, a recording stage 258 and a further
processing stage 260. Each of these stages is described below.
[0033] The wireless communication apparatus 200 also comprises a
digital-to-analogue converter (DAC) 270 having an input coupled to
the processing stage 250 for receiving signals to be transmitted.
An output of the DAC 270 is coupled to an input of a transmitter
280 for converting signals into a suitable form for transmission,
and an output of the transmitter 280 is coupled to an input of the
duplex filter 220 for coupling the signals for transmission to the
antenna 210.
[0034] An input of the block formation stage 251 is coupled to the
input of the processing stage 250 for receiving the samples of the
composite signal, and the block formation stage 251 divides the
samples of the composite signal into a plurality of blocks. Each of
the blocks comprises a number of the samples corresponding to not
more than half of the tone burst duration. Therefore, for the case
of GSM modulated carrier signals, where the FCCH duration and tone
burst duration is 0.577 ms, and an LTE receiver operating with a
sample rate of 30.72 M samples/sec, each tone burst corresponds to
17725 samples, and in one embodiment a convenient block size is
8192.
[0035] An output of the block formation stage 251 is coupled to an
input of the transform stage that transforms the samples of each
block into frequency domain components. The transform stage may
implement, in one embodiment, an FFT, and where the block size is
8192, this may be an 8192-point FFT, which provides 8192 frequency
domain components corresponding to 8192 spectral bins of the FFT.
An output of the transform stage 252 is coupled to an input of the
group formation stage 253 that divides the frequency domain
components of each block into a plurality of groups, each group
corresponding to a range of frequencies occupied by a different one
of the modulated carrier signals. Therefore, for the case a sample
rate of 30.72 M samples/sec and an 8192-point FFT, the frequency
domain components are separated by 3.75 kHz, and a GSM modulated
carrier signal which occupies a bandwidth of 200 kHz will comprise
53 frequency domain components, occupying 53 FFT spectral bins. An
output of the group formation stage 253 is coupled to an input of
the tone burst detection stage 254.
[0036] The tone burst detection stage 254 comprises a normalization
stage 255 having an input coupled to the input of the tone burst
detection stage 254, a convolution stage 256 having an input
coupled to an output of the normalization stage 255, and a decision
stage 257 having an input coupled to an output of the convolution
stage 256 and an output coupled to an output of the tone burst
detection stage 254. The normalization stage 255 normalizes the
frequency domain components of each group, which may be performed
by dividing each frequency domain component within the group by the
RMS value of all of the frequency domain components in the group.
The convolution stage 256 generates the frequency domain indication
of signal autocorrelation, R(f), for each group by evaluating the
expression in equation (1). The decision stage 257 determines those
groups that have a strong DC component by comparing, for each
group, the frequency domain component at DC with the average value
of all other frequency domain components in the group. In
particular, the decision stage 257 may calculate the ratio of the
frequency domain component at DC to the average value of all other
frequency domain components in the group, which may be expressed as
R(0)/average(R(f.noteq.0)), and compare the ratio with a threshold.
Those groups for which the ratio exceeds the threshold are deemed
to be modulated with the tone burst. An output of the tone burst
detection stage 254, and therefore the output of the decision stage
257, is coupled to an input of a recording stage 258 which records
an indication of those groups that are deemed to be modulated with
the tone burst. An output of the recording stage 258 is coupled to
an input of a further processing stage 260.
[0037] The further processing stage 260 comprises an RSSI stage 261
having an input coupled to the input of the further processing
stage 260, a frequency error stage 262 having an input coupled to
an output of the RSSI stage 261, and a framing stage 263 having an
input coupled to an output of the frequency error stage 262. An
output of the framing stage 263 may be coupled to further,
non-illustrated stages that are not material to the present
disclosure. The RSSI stage 261 determines, from the frequency
domain components of the group corresponding in frequency to each
of the carrier signals determined to be modulated with the tone
burst, an RSSI. For example, the RSSI stage 261 may determine the
RSSI as the mean square value of the frequency domain components
within a group, without normalization. The frequency error stage
262 estimates a frequency error of the local oscillator of the
receiver, relative to one or more of the modulated carrier signals.
This frequency error, which may be evaluated with a precision equal
to the frequency spacing between the frequency domain components in
each group, is expressed in equation (2) for the case of GSM. The
framing stage 263 determines, for the case of a GSM modulated
carrier signal, the GSM frame and slot boundaries, with a precision
equal to one block duration, that is, half the duration of the
FCCH, according to which of the blocks the tone burst is detected
in. The wireless communication apparatus 200 may employ the RSSI
for selecting among modulated carrier signals in further stages of
cell selection, may employ the frequency error for correcting its
local oscillator frequency, and may employ the determined frame and
slot boundaries for decoding system information conveyed by the
modulated carrier signals.
[0038] The processing stage 250 may comprise a processor operating
under the control of computer program code. The processor may
comprise a sequential state machine operative to execute machine
instructions stored as machine-readable computer programs in
memory, such as one or more hardware-implemented state machines
(e.g., in discrete logic, FPGA, ASIC, etc.); programmable logic
together with appropriate firmware; one or more stored-program,
general-purpose processors, such as a microprocessor or Digital
Signal Processor (DSP), together with appropriate software; or any
combination thereof.
[0039] The processing stage 250 may comprise a data storage device
storing computer program code arranged to perform, when executed on
a processor, the operations described herein as being performed by
any or all of: the block formation stage 251, the transform stage
252, the group formation stage 253, the normalization stage 255,
the convolution stage 256, the decision stage 257, the recording
stage 258, the RSSI stage 261, the frequency error stage 262 and
the framing stage 263. Alternatively, any one or more of these
stages may be implemented in hardware, such as in an ASIC block,
with appropriate interconnection to the processor executing one or
more other stages as software modules.
[0040] Such computer program code, and other data, may be carried
by any nontransient machine-readable media known in the art or that
may be developed, including but not limited to magnetic media
(e.g., floppy disc, hard disc drive, etc.), optical media (e.g.,
CD-ROM, DVD-ROM, etc.), solid state media (e.g., SRAM, DRAM, DDRAM,
ROM, PROM, EPROM, Flash memory, etc.), or the like.
[0041] The performance of an exemplary method disclosed herein has
been evaluated by means of computer simulation. Referring to FIG.
3, the probability of correct detection of the tone burst is
plotted for a range of signal to noise ratios. It can be seen that
the tone burst may be reliably detected for signal to noise ratios
exceeding about -20 dB. The probability of falsely detecting the
tone burst was evaluated as zero across the same range of signal to
noise ratios.
[0042] Referring to FIG. 4, the magnitude of the 53 frequency
domain components corresponding to a GSM carrier signal modulated
with an FCCH and having a signal to noise ratio of -20 dB is
plotted. A distinct peak is visible at the nineteenth frequency
domain component, corresponding to the nineteenth spectral bin
which covers a frequency range 67.5 kHz to 71.25 kHz, which
indicates the presence of the tone burst in the nineteenth
frequency domain component, resulting from the tone burst having a
frequency of 67.71 kHz relative to the center frequency of the GSM
carrier signal.
[0043] Referring to FIG. 5, the frequency domain indication of
signal autocorrelation, R(f), is plotted for the 53 frequency
domain components corresponding to a GSM carrier signal modulated
with a BCCH and having a signal to noise ratio of -20 dB. A
distinct peak is visible at DC--that is, at the first spectral
bin--indicating the presence of the tone burst. For comparison with
FIG. 4, FIG. 6 shows the magnitude of the 53 frequency domain
components corresponding to a GSM carrier signal which is not
modulated with an FCCH, for a signal to noise ratio of -20 dB. It
can be seen that there is no distinct peak visible in FIG. 6
corresponding to a tone burst. Likewise, for comparison with FIG.
5, FIG. 7 shows the frequency domain indication of signal
autocorrelation, R(f), plotted for the 53 frequency domain
components corresponding to a GSM carrier signal which is not
modulated with a BCCH, for a signal to noise ratio of -20 dB. No
distinct peak is visible at DC in FIG. 7, because of the absence of
the tone burst.
[0044] Although the use of an LTE receiver for receiving GSM
carrier signals modulated with an FCCH has been described for the
purpose of illustration, other types of receiver may alternatively
be used and other types of modulated carrier signals modulated with
other types of tone burst may be received and detected.
[0045] Although a tone detection algorithm based on the frequency
domain indication of signal autocorrelation defined by equation (1)
has been described, other frequency domain tone detection
algorithms may alternatively be used.
[0046] Although transformation of the samples of the composite
signal to the frequency domain components has been described based
on the use of an FFT, other types of transform from the time domain
to the frequency domain may alternatively be used.
[0047] Other variations and modifications will be apparent to those
of skill in the art. Such variations and modifications may involve
equivalent and other features that are already known and which may
be used instead of, or in addition to, features described herein.
Features that are described in the context of separate embodiments
may be provided in combination in a single embodiment. Conversely,
features that are described in the context of a single embodiment
may also be provided separately or in any suitable
sub-combination.
[0048] It should be noted that the term "comprising" does not
exclude other elements or steps, the term "a" or "an" does not
exclude a plurality, a single feature may fulfill the functions of
several features recited in the claims and reference signs in the
claims shall not be construed as limiting the scope of the claims.
It should also be noted that the Figures are not necessarily to
scale; emphasis instead generally being placed upon illustrating
the principles of the present invention.
[0049] The present invention may, of course, be carried out in
other ways than those specifically set forth herein without
departing from essential characteristics of the invention. The
present embodiments are to be considered in all respects as
illustrative and not restrictive, and all changes coming within the
meaning and equivalency range of the appended claims are intended
to be embraced therein.
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